Soap Tutorial Part II: The Recipe

In Part I: The Fundamental Components, we covered the necessary and optional ingredients of a soap recipe. This Part II: The Recipe covers how to design a recipe and calculate measurements. Part III: The Process, will go over the supplies and how to put it all together.

A small batch of my standard recipe containing castor oil, cocoa butter, olive oil, palm kernel oil, and mango butter.
A small batch of my standard recipe containing castor oil, cocoa butter, olive oil, palm kernel oil, and mango butter.

Everything starts with the oils. As covered in Part I, it is possible to make single-oil soaps. Lard, coconut oil and palm oil are all popular options. Most soapers enjoy devising different combinations, however. They do this by perusing breakdowns of the qualities that different oils/fats/butters will impart to the soap.

How a particular combination of oils will work together can be tested on tools like this one at, which predicts how the soap will turn out on ranges of qualities such as hardness, cleansing, conditioning, bubbly, etc. Play around with different percentages of the oils you want to use, tinkering with the proportions, until you get the qualities you want.

Alternatively, you can use a recipe already created by someone else. In this case, I am giving you one of my standard combinations:

Castor Oil               7%
Cocoa Butter         10%
Olive Oil                48%
Palm Kernel Oil    25%
Shea Butter            10%
Total                      100%

But those are percentages, not measurements! That brings us to the second step.

Mold Volume = Total Combined Oil Weight. The size of a batch is dictated by the volume of the available mold(s).

There are a variety of soap molds available for sale, loaf molds, slab molds, soap molds with divides, wood molds, acrylic molds, plastic molds, etc. You don’t have to buy one though.

You can just use what you have on hand. A meatloaf or bread pan. A cupcake pan (use cupcake liners or it will be hard to get the pieces out). A cake dish. A pizza box or shoebox. Whatever ya got. Just keep the following in mind:

  • Generally you shouldn’t use aluminum for soap making because lye will react with it. If you’re hot processing your soap, it isn’t molded until the lye is fully neutralized. So if you’re certain you got it fully cooked, it should be okay to use aluminum, right? If you’re not convinced, use stainless steel or ceramic or stoneware or a cardboard box.
  • If you’re using a purchased mold, follow the directions that came with it as to whether it needs to be lined. If you’re using something that you have on hand, line it with freezer paper or parchment paper (or cupcake wrappers as the case may be) so that it is easier to get the hardened soap out. Google “how to line a soap mold” for various tutorials on how to do this most efficiently.

If you want the recipe to fit into a specific mold, you have to determine the volume that mold will hold. A purchased soap mold will specify.

If you are using a mold of unknown volume, Google “how to calculate the volume of a soap mold. What I have done in the past is: 1) calculate the volume of the mold in cubic inches by multiplying the length by the width by the height; 2) multiply the resulting volume by .40.

This is the total combined amount of oils in ounces to use in the recipe. The percentages allocated to each oil by playing around with the SoapCalc tool (or following my recipe) are percentages of whatever volume in total oils your mold is designed to hold.

NOTE: You might be thinking, “What about the lye and liquid? Don’t those have to fit too?” I can’t explain chemically or mathematically why that you don’t need to worry about them—but I never have and neither do most of the soapers I follow.

I have an acrylic mold with dividers that I ordered online from Soap Making Resource. It was designed to hold 5 pounds of soap. I find this results in bars so thick they are clumsy to use at first. Two and a half pounds results in reasonably sized bars. Obviously, it will accommodate anything between those two.

One of my small loaf molds, lined with freezer paper and filled with freshly cooked hot-processed soap.
One of my small loaf molds, lined with freezer paper and filled with freshly cooked hot-processed soap.

I also have two smaller loaf molds purchased from Michaels (I used coupons to get steep discounts on these). I see now that they are specified to hold 3 pounds of soap. With hot process, you can pile your (mashed potato consistency) soap up above the level of the mold. Still, even taking that into account, I’m skeptical that 3 pounds of soap would fit comfortably into these molds. I’ve always used the calculation described above to come up with a smaller amount: 9 inches long x 3 ¾ inches wide x 2 ¼ inches high = 75.9375 x .40 = 30.375 ounces (1.9 pounds).

I also have a loaf baking dish that I sometimes use (all of them except the acrylic one get lined with parchment or freezer paper) that works out like this: 9 inches long x 5 inches wide x 3 ¼ inches deep = 146.25 x .40 = 58.5 (3.66 pounds).

So here’s how the percentages of oil would convert to measurements for each of my mold volumes:

Oil              Percent        1.9 lbs.         2.5 lbs.         3.66 lbs.       5 lbs.
Castor            7 %           2.13 oz.        2.8 oz.          4.10 oz.        5.6 oz.
Cocoa B.      10 %           3.04 oz.       4.00 oz.       5.86. oz.       8.00 oz.
Olive Oil     48 %          14.59 oz.     19.20 oz.     28.11 oz.        38.40 oz.
PKO             25 %           7.60 oz.      10.00 oz.     14.64 oz.      20.00 oz.
Shea B.        10 %           3.04 oz.        4.00 oz.       5.86 oz.        8.00 oz.

As you can see, soap recipes are often about percentages rather than about measurements. The measurements change depending on what volume you want to achieve based on your mold(s).

Now. What about the lye and liquid and other things?

As discussed in Part I, all but the lye are based off the oil amounts. The lye is the only complicated one. We’ll tackle that now.

Calculating the Lye. Don’t worry—you don’t have to do any of the math if you don’t want! There are various online calculators that will determine lye amounts for your recipe in a matter of moments. All you have to do is tell the calculator how much soap you want to make and what oils (in percentages or weights, depending on the particular calculator) you are planning to use.

You should at least know the theory behind the math, though.

This will prevent you from doing anything silly (and dangerous) like finding a recipe on the Internet, deciding to substitute an oil you have on hand for one in the recipe, and then using the same lye measurement as was given in the original recipe.

In fact, I never accept the lye weight given in a recipe designed by someone else—even if I’m not making changes to the oils. The reason is that if there is an error in the calculation, at best you might waste ingredients on a failed batch of soap (lye heavy soap can be recycled into laundry soap—but that’s another tutorial). At worst, you might burn yourself or someone else with caustic soap.

Some websites won’t even give lye weights for their recipes for this reason; they will just tell you to go run the oils through a lye calculator.

Even if they do give it, always double check the lye weight using at least two online lye calculators. If you do want to  know how to do the math yourself:

Each different fat/oil/butter has its own individual “saponification value.” That is the amount of lye in milligrams required to “marry” all of the molecules in one gram of that fat.

NOTE: Official saponification values are expressed in milligrams of potassium hydroxide. Sodium hydroxide is used in bar soap. Official sap values have to be converted when using sodium hydroxide (unless they have already been converted; be sure to check) by dividing the KOH value by the ratio of the molecular weights of KOH and NaOH (1.403). Of course, if you use an online lye calculator, you simply tell it which kind of lye you are using and it takes care of the calculations.

There are lists of oil saponification values on the web. Many have sap values in both KOH and NaOH, so there is no need to convert. There are variations between the lists. This is because ingredients aren’t standardized—they differ from source to source and even batch to batch. The lists are compiled using an average of reported from values from different sources. Research each list and decide which to use as a default. How to deal with the unavoidable uncertainty is discussed below.

For purposes of illustration only, I am using the following as ap values for my list of oils:

Oil                                KOH                            NaOH
Castor                           .18                                  .128
Cocoa Butter               .1918                              .137
Olive Oil                      .1876                              .134
Palm Kernel Oil         .2184                              .156
Shea Butter                 .1792                              .128

(Some of these are a little low in comparison with values given on some lists. That’s ok. We’re just using this for illustration purposes and, in connection with online calculators, to generate a range of lye weights.)

These numbers tell you how much of that type of lye you need to saponify each oil using consistent units of measurement. So .128 of an ounce of sodium hydroxide is how much you need to saponify an ounce of castor oil. Or .134 of a gram of sodium hydroxide will saponify one gram of olive oil. Or .156 of a pound of sodium hydroxide will saponify a pound of palm kernel oil.

Let’s apply those values to my recipe to calculate the total lye weight for a 1.9-pound batch of soap (because you should start small):

Oil              Percent        1.9 lbs.           Sap Value    Lye Needed
Castor           7 %              2.13 oz.          .128              0.2726
Cocoa B.      10 %            3.04 oz.          .137              0.4165
Olive Oil     48 %           14.59 oz.          .134               1.955
PKO             25 %            7.60 oz.          .156               1.1856
Shea B.        10 %            3.04 oz.          .128              0.3891
                                                         TOTAL LYE        4.2188

Setting aside the uncertainties mentioned above, we need 4.2188 ounces of sodium hydroxide to make a 1.9-pound our recipe using the percentages of oils we have chosen.

Now we need to consider the implications of those aforementioned uncertainties. We know that the ratio is not going to be exact because we don’t know the specific values of our individual batches of oils and we don’t know the precise impurity level of our lye. Would we rather err on the side of too much or too little lye?

Let us consider the possibilities: 1) heavy on the lye; 2) exactly even; or, 3) heavy on the fat.

Heavy on the lye. All of the fat molecules get married. But some of the lye molecules are still hanging out, single, watching Netflix and collecting cats. Being lye-heavy can make for an extra-cleansing soap. Some people intentionally make it that way to use in a clothes washer, for example.

Lye-heavy soap can be unpleasant or dangerous (depending on how lye heavy) for skin use, however. For skin soap, it is best if all the molecules of lye get married to a nice fat person so their lye aunts and uncles can stop pestering them already.

Which leads to one of the other two possibilities:

Perfectly balanced. I have mainly seen exact ratios of lye-to-fat come up in liquid soap-making for one of two reasons: the soap-maker wants the soap to be maximally cleansing or the soap-maker is attempting to make perfectly clear liquid soap (free oils will turn the liquid soap cloudy).

As already mentioned, due to impurities in and differences between individual batches of ingredients, it is all but impossible to get the ratio exactly right. There will be a chance of unmarried lye left at the end. People who do these techniques have ways of counteracting that (such as by adding some other acid at the end, like a citrus, to neutralize any remaining lye).

I don’t know a whole lot about these techniques. I don’t have any particular need for clear liquid soap and have never attempted to make it that way. I have painfully dry skin, so I have always been more interested in making soap moisturizing than clear. Which brings us to the third option.

Fat heavy. In this approach, you “super-fat” or “lye discount,” which means using less lye (commonly by 5-8%, but some people take it further) than would be needed to saponify all the fat. This ensures that all the lye molecules get married. It also makes for a more moisturizing final product (because some of the oils are left unattached to engage in heavy make-out sessions with your skin). It won’t be quite as cleansing though.

NOTE: Theoretically, you can influence which fat’s particles remains free. The way to do this is to reserve some portion or all of that oil/fat/butter from the mixture. Pour the lye-liquid in without that ingredient added yet. Bring the mixture to trace (emulsification, discussed in Part III). At that point, all the other oils are married. Now add your reserved ingredient. Some of it will enter into a shotgun union with whatever free lye remains. The rest will be the “super-fat” that remains at the end. (If it were me, I’d choose one of the butters for this.)

I generally make my recipes with 7-8% super-fat/lye discount without worrying about which oils remain. For purposes of illustration:

4.2188 x .92 (a 8% discount or 8% superfat) = 3.88 ounces of sodium hydroxide.

Recall from Part I that I calculate my water as 38% of total oils (1.9 lbs.) for a measurement of 11.55 ounces of water in which to dissolve the lye. Other people use 2x or 3x their lye weight as the water measurement.

Now let’s check our results by running the same recipe through a couple of online soap calculators. The two I use most frequently are the one at and the one at Majestic Mountain Sage.

SoapCalc tells me I need 4.05 ounces of lye and 11.55 ounces of water.

MMS gives me 3.99 ounces of lye and 8-11 ounces of water. In Part I, I mentioned that I sometimes use sodium lactate in the lye liquid. I see that MMS gives a range for sodium lactate in ounces (.3-.91 ounces) rather than teaspoons and directs me to use the high end of the water range if I am going to use the SL (which I would do anyway given my high altitude).

So we can see that both SoapCalc and MMS came out with higher lye weights than I came up with myself. That is probably because I was using sap values from a list that had consistently lower values than other lists.

Because I trust both SoapCalc and MMS, and because I am not averse to heavy superfatting, I would feel comfortable using any amount of lye between my own calculation on the low end and SoapCalc’s on the high end. Most likely, I would go with the middle number and use the measurement given by MMS.

Now we are ready to put our recipe together. I am giving the measurements for a 1.9-pound recipe. Hopefully you now understand how to modify this for any other volume you wish to make:

Oil              Percent         1.9 lbs.
Castor            7 %            2.13 oz.
Cocoa B.      10 %            3.04 oz.
Olive Oil     48 %           14.59 oz.
PKO             25 %             7.60 oz.
Shea B.        10 %             2.13 oz.

Liquid 11.55 ounces of distilled water (of which I will reserve two for the sugar)
Lye 3.99 ounces
Sodium Lactate 0.30-0.91/1 teaspoon OR table salt at 1 teaspoon

Sugar 2 teaspoons (dissolved in reserved distilled water)

Essential Oils 1.43 ounces
Arrowroot in a magical, unspecifiable amount that just feels right

You’ve reached the home stretch. Now all you need are the supplies and the technique. I’ll cover both in Part III: The Process.

For Part I: The Components, click here.
For Part III: The Process, click here.

3 thoughts on “Soap Tutorial Part II: The Recipe

  1. Pingback: Soap Tutorial Part I: The Fundamental Components | Hashtag Sarah

  2. Pingback: Soap Tutorial Part III: The Process | Hashtag Sarah

  3. Pingback: Tutorial: Hot Processed Salt Soap Bars | Hashtag Sarah

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